If we're fortunate -- as more and more people increasingly are throughout history -- we'll live for a long time. And we'll learn many things over this long lifetime, mostly from experience. Considering that lifetimes are so long, here are some things that I've learned from experience that are beneficial to master early on in life (say in your teens and twenties) that will compound throughout your life and increase in value exponentially.
Breaking Things Down -
Reductionism, or being able to explain or do something by reducing its complexity such as designing a bridge by analyzing each joint, is a valuable skill to master because it can be applied to so many possible avenues of life.
In science, mastering first principles can translate to understanding basics in other industries. For example, from physics to policy: Newton's 1st law of motion says an object in motion or at rest will continue doing what it is doing unless acted on by an outside force => the status quo will remain the status quo unless supplanted by a different idea or policy, it's not just going to improve by itself. The bigger the existing policy, the more inertia it has and the more force it's going to take to replace the policy.
In writing and linguistics, knowing how to break words down into their root meanings can help you identify it. I took the GRE 10 years ago and again this year, and my verbal score drastically improved without direct study from just reading a lot but also from learning to break down words into their roots. For instance, the word "unequivocal"; while not hard to understand can be confused from time to time, but by knowing "un-" means 'not', "-equi-" means equal, and "-vocal" means to talk about, I can trace back the words meaning into "not being equal to talk about" or something that is unequivocal having no other equals to be talked about.
In math, to solve a complicated multi-step problem, you're taught to solve it by breaking it down into many smaller problems that are nested and connected.
Perfecting Your Signature -
When I was in 5th grade, a friend and I were convinced we were going to be famous baseball stars (he got a lot closer in the end by actually making the HS baseball team and eventually winning a college football championship). So naturally we needed to perfect our signatures at the age of 10. I noticed from my memorabilia that signatures were really mostly just a standout first and last letter with scribbles in between. So I created, through much practice I might add, a signature with a C-y J-l. Little did I know some 20 plus years later that creating a simple and efficient signature early on would help me easily sign things thousands of times since then, with many more to come.
Simple Math Skills -
Learning basic multiplication examples (20 times a number is just doubling it and adding a zero at the end, 1 through 12 times 1 through 12, the 20% trick when tipping as a baseline, etc.) is useful many times throughout your life. And while we'll always have calculators at our disposal, simple mental calcs are incredibly useful and time-saving. One of the most important things to master is rounding and estimating -- it's a simple skill and "just good enough" but used more than finding exact answers by orders of magnitude in everyday life. Other topics that have outsize influence on your life include exponential growth and decay, rates of change, basic statistics, etc.
Taxes -
The earlier that you understand the basics of how taxes work (income tax rate brackets, short-term vs. long-term gains, write-offs, etc.), the earlier you can maximize your benefits of the tax system in your career, which should compound in both monetary and knowledge value throughout your life.
Improving at Driving -
Oh man, driving. Driving is one of the most mundane yet dangerous things that we do. And yet, many drivers don't consciously try to improve their driving after becoming comfortable with the skill in their teen years. For me, it took a couple near misses or mistakes to convince me to try and improve. I think the biggest thing that I've improved on is being a proactive driver instead of a reactive one -- not looking at the car in front of me but through it to the car in front of them, anticipating the the car to your front right or left that leveled off their speed to match yours and the car in front of you and will cut you off, even walking I'll wait the extra second before crossing because I've almost been hit multiple times since drivers forget right-of-way rules. Improving at driving from your teen years through your first 10-15 years of driving will pay off in a safer experience, less money wasted on accidents, repairs, and insurance, and will reduce the stress of driving throughout your life.
Establishing a Regular Exercise Routine -
Establishing a regular exercise regimen in your life is one of the best things you can do for your physical (and mental) well-being. Most people concentrate on absolutes (I want to lose X pounds or be able to lift Y weight), but by focusing on relative values (I weigh X pounds more or less than yesterday, it's because of these things, or I was able to lift Y more or less weight for these factors) is more important. But the most important thing to do in be consistent, as this will push you through the hard times that you will inevitably encounter in your exercise program. Just be consistent. As James Clear says, the heaviest weight in the gym is the front door.
Understanding Nutrition and Getting Good at Cooking -
Fad diets will always come and go, but establishing a baseline nutrition guide is crucial to your long-term health. Almost as important, and much more enjoyable, is getting good at cooking early on in your adult life. Over your twenties you should master the basics of cooking and then from there you can improve at will. But knowing how to cook good scrambled eggs or how to make a steak medium rare versus medium will never go out of fashion. Plus the better you get at it earlier, the more fun cooking becomes and the more you'll want to do it versus eating out, thus saving you a lot of money in the long run.
Understanding Your Personality Traits -
There are arguments for and against personality tests. They're pretty much useless when it comes to predicting how different people will work together in teams and what not. However, I believe they do hold a lot of value in describing parts of your personality that maybe you couldn't put into words before (provided you got regular results on the tests over time, which is debatable). Unlike astrology where the root of the argument is basically when your parents had sex and conceived you determines your personality, tests like the Big 5 or Myers-Briggs should be taken with a grain of salt but can explain things that others similar in personality to you have found helpful. To me, when I first discovered them 8-10 years ago, it was like a light bulb went off and has influenced my thinking every day since then.
Determining they're, their, there; to, too, two; who's, whose; etc. -
These mistakes are way too common in everyday life to neglect and never improve on. For me, I always do a double check to make sure I used the correct one when using similar sounding words (although I definitely still miss some). Really it just takes conscious practice and review to improve. It took me way too long to properly use "who's" versus "whose", but an easy tip is "who's" can be answer by he/she/them and "whose" by his/hers/theirs.
Improving Your Typing Speed -
I remember hating that I was signed up for a semester long typing course early on in high school. I had been able to type via pick and choose and looking at the keyboard, was this class really necessary? One could argue it has been one of the most beneficial classes that I took in high school or ever, in terms of practicality and amount of time saved throughout my life thus far. Learn to type properly early on, even if it is slower in the short-term, because the benefits will compound over time to be massive in the long-term.
Writing Your Thoughts Regularly -
I hated writing in school, with a passion. Only recently have I seen the benefits of what writing can do to clarify your thinking. One of the best things you can do early on, similar to an exercise or nutrition program, is establish a regular program of writing your thoughts down. It can be in your phone's notes, in a journal or diary, or a public blog. But writing is so very helpful to thinking and communicating clearly that this regular program will have invaluable returns throughout your life.
Communicating Clearly Electronically -
When we communicate with others and are not face-to-face, we miss out on all the non-verbal cues that make up effective human communication. So when responding to emails or texts, it's best to be clear and concise. This is more important in professional settings, as miscommunications can have time and money repercussions along with relationships. One of the best tips I've learned is getting to the point early in the communication. For example, when sending a well thought out email, bullet point highlight the two to three most important things in the email at the top, so their importance isn't lost in the body of text. Use bold and highlight to also show importance. These tips go a long way in providing direct and clear communication.
Using a Calendar and its Reminders for Future Events -
Time management is such a crucial skill to learn early on. One of the best things you can start doing is inputting future events into your phone's calendar and setting reminders for them for a couple hours or days before. When my life was busiest, if something wasn't added to my calendar than I missed it, as I didn't have the mental capacity to remember when events were supposed to happen. Agree to a lunch with a coworker tomorrow? Set an event for it tomorrow and a reminder in the morning. Set a recurring reminder to call your mom and dad every week. Electric bill due on the 15th? Set a reminder to pay it on the 12th. It seems women intuitively get this as a lot of them that take birth control have a regular reminder alarm that goes off at the same time every day.
Getting the Right Timing -
Very related to the above topic, and something I plan on delving much deeper into in a longer blog post, is the importance of timing. Whether it's relationships, job changes, sports, entertainment, etc. almost everything in this world is highly dependent on timing. You meet someone great but are in a happy relationship, so you stay friends. But had your relationship been in another state or you met the other person at a different time, things could be drastically different. When it comes to acting on those reminders mentioned above, timing becomes so much more crucial. Notice when I mentioned setting the reminder about lunch with your coworker for the morning, so that way you won't pack a lunch that day. Or setting the bill reminder for a couple days before, because if it pops up and you're not able to act on it immediately, you can snooze until tomorrow and still pay it on time. For me, I've been setting reminders to go off at times when I think I will be able to act on them, so I set bill payments for at night when I'll be home and near my computer rather than during the day when I'm out and about. Don't send tweets or messages late at night if your goal is to get the most eyeballs on it, as most won't check their feed the next morning to see all that they missed while sleeping.